1. Field of the Invention
The present invention relates generally to air cooled turbine airfoils, and more specifically to a film cooling hole for the airfoils.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In a gas turbine engine, a turbine comprises a number of stages of stator vanes and rotor blades used to convert the energy from a hot gas flow into mechanical energy used to drive the rotor shaft. The efficiency of the engine can be increased by passing a higher temperature gas flow into the turbine. However, the highest temperature allowable is dependent on the material properties of the first stage airfoils (vanes and blades) and the amount of cooling provided. Once the material properties have been established, higher temperatures can be used if adequate cooling of the airfoils is provided.
Current airfoil cooling designs make use of internal convection and impingement cooling, and film cooling of the external airfoil surfaces that are exposed to the high temperature gas flow. Film cooling provides a blanket of cooling air over the airfoil surface that—in theory—prevents the hot gas flow from making contact with the airfoil surface. One major objective of a turbine airfoil designer is to maximize the effect of the cooling air while minimizing the usage of the cooling air in order to increase the efficiency of the engine, since the pressurized cooling air used for cooling the airfoils is bled of from the compressor of the engine. The bled off cooling air becomes wasted work.
Prior art film holes pass straight through the airfoil wall at a constant diameter and exit at an angle to the airfoil surface. Some of the cooling air is consequently ejected directly into the mainstream hot gas flow and causing turbulence, coolant dilution and a loss of downstream film effectiveness. Also, the hole breakout in the stream-wise elliptical shape will induce stress problems in a blade application. FIG. 1 summarizes this particular film hole design.
FIG. 2 shows a standard 10×10×10 stream-wise three dimension diffusion hole of the prior art. This type of film cooling hole comprises a constant cross section flow area at the entrance region for the purpose of metering the cooling flow. Downstream from the constant diameter section, the cooling hole is diffused into three directions. However, there is no diffusion in the upstream corner of the film cooling hole in the stream-wise direction as indicated by the top surface of the film hole in FIG. 2b. During engine operation, hot gas frequently becomes entrained into the upper corner (hot gas injection zone 12) and causes shear mixing with the cooling air. As a result, a reduction of film cooling effectiveness for the film hole occurs. In addition, internal flow separation 13 occurs within the diffusion hole at the junction between the constant cross section area and the diffusion region as shown in FIG. 3.